As an instrument for sensing a trace substance in a solution or gas, there has been known a sensing instrument which uses QCM (Quarts Crystal Microbalance) formed of a piezoelectric resonator, for example, a quartz-crystal resonator. A sensing instrument of this type senses a trace substance being a substance to be sensed by making the quartz-crystal resonator adsorb the trace substance and finding a change in its oscillation frequency (resonant frequency). The present applicant has developed such a sensing instrument and expects that in future in various fields, it will become possible to sense, for example, dioxin being an environmental contaminant, specific antigens in blood or serum, and the like on an extremely low concentration level, for example, down to a ppb to ppt level.
Since the oscillation frequency of the quartz-crystal resonator used in the sensing instrument changes depending on temperature, some measure has to be taken in an oscillator circuit utilizing the quartz-crystal resonator in order to obtain frequency stability to temperature. Regarding this, there has been proposed a quartz-crystal resonator of a so-called twin sensor type that does not require a large-scale instrument structure such as an OCXO (Oven Controlled Crystal Oscillator) conventionally used, and is as compact and light-weighted as a TCXO (Temperature Compensated Crystal Oscillator) yet has higher frequency stability than the TCXO (for example, patent document 1 and patent document 2).
FIG. 5 is a structure example of a quartz-crystal oscillator 10 mounted in the sensing instrument developed by the present applicant, and the quartz-crystal oscillator 10 includes: a quartz-crystal resonator 1 of a twin sensor type structured such that excitation electrodes 101 to 103 are provided on a quartz-crystal piece 100 having two different oscillation areas (a first is oscillation area 105 and a second oscillation area 106) elastically insulated by an elastic boundary layer 107 being an elastic boundary area; and a pair of Colpitts oscillator circuits 111, 112 connected in series to the oscillation areas 105, 106 in order to take out oscillation frequencies from the oscillation areas 105, 106. For example, the excitation electrodes 101 to 103 are different in mass or size or the oscillator circuits 111, 112 are different in load capacitance, which makes it possible to obtain frequency signals with different oscillation frequencies from the respective oscillation areas 105, 106.
An adsorption layer capable of adsorbing a substance to be sensed is provided on the excitation electrode 103 on one of the areas (for example, the second oscillation area 106) of the quartz-crystal piece 100 and is capable of adsorbing a trace substance, and a block layer not adsorbing the substance to be sensed is provided on the excitation electrode 101 on the other area (for example, the first oscillation area 105). When a sample solution is supplied to a front surface of the quartz-crystal piece 100 on which these adsorption layer and block layer are formed, the substance to be sensed is selectively bonded with the adsorption layer to change the oscillation frequency of the second oscillation area 106 side on which the adsorption layer is formed. Hereinafter, an output of the first oscillation area 105 side will be called a channel 1 and an output of the second oscillation area 106 side will be called a channel 2. By subtracting the oscillation frequency of the channel 1 side from the oscillation frequency of the channel 2 side which has been influenced by the adsorption of the substance to be sensed and comparing the results before and after the adsorption, it is possible to find a variation in the oscillation frequency that is ascribable to the adsorption of the substance to be sensed.
As described above, the oscillation frequency of the quartz-crystal resonator changes according to a change in an ambient temperature, and here the principle for giving the quartz-crystal resonator 1 of the twin sensor type frequency stability to the temperature change will be described. A frequency-temperature characteristic 7b shown in FIG. 18 shows how the oscillation frequency in the second oscillation area 106 in the state of not adsorbing the substance to be sensed changes in accordance with a temperature change. When the frequency-temperature characteristic 7b of the second oscillation area 106 presents such a change, a variation in the oscillation frequency shown by a frequency-temperature characteristic 7a of the first oscillation area 105 provided on the common quartz-crystal piece 100 is substantially the same as that shown by the frequency temperature characteristic 7b of the second oscillation area 106 side as shown in FIG. 18.
Therefore, even when the oscillation frequencies from the first oscillation area 105 and the second oscillation area 106 change due to the change in the ambient temperature, a difference between the oscillation frequencies (frequency difference) of the both channels is substantially constant and does not change. Therefore, by comparing the frequency differences before and after the adsorption of the substance to be sensed, it is possible to highly accurately find a variation in the oscillation frequency caused by the adsorption, from which the influence of the frequency-temperature characteristic is removed.
After conducting various studies with the intention of achieving the practical application of a sensing instrument having high stability to, for example, a temperature change by using the quartz-crystal resonator 1 of the twin sensor type described above, the present inventors have found out that the following problem arises when the frequency difference between the two channels is too small or too large. Specifically, it has been found out that, when the oscillation frequencies of the first oscillation area 105 and the second oscillation area 106 are set to close values to thereby make the frequency difference therebetween small, the frequency difference between a frequency-temperature characteristic 71a of the first frequency area and a frequency-temperature characteristic 71b of the second frequency area does not become constant and the frequency difference unstably changes in accordance with a change in the ambient temperature as shown in FIG. 19, for instance. A possible reason for this is that, when the frequency difference between the both channels is reduced by setting the oscillation frequencies of the first oscillation area 105 and the second oscillation area 105 to close values, the two oscillation areas 105, 106 come into a state of being elastically coupled even though the elastic boundary layer 107 is provided. Such unstable change in the frequency difference not only makes it difficult to accurately find a variation in the oscillation frequency caused by the adsorption of the substance to be sensed but also lowers a function for accurately finding the variation in the oscillation frequency caused by the adsorption of the substance to be sensed because due to the elastic coupling of the two oscillation areas 105, 106, a change in an oscillation state of the second oscillation area 106 side caused by the adsorption of the substance to be sensed also changes the oscillation frequency of the first oscillation area 105 side not sensing the substance to be sensed.
When the oscillation frequencies of the first oscillation area 105 and the second oscillation area 106 are set greatly different from each other and thus the frequency difference is made large, the frequency difference between the frequency-temperature characteristic 72a of the first oscillation area 105 and the frequency-temperature characteristic 72b of the second oscillation area 106 tends to gradually increase as shown in FIG. 20, for instance; therefore, in this case, the method of canceling the influence of the frequency-temperature characteristic by finding the difference between the oscillation frequencies of the both channels cannot be employed either. The patent document 1 and the patent document 2 mentioned above do not find these problems that the quartz-crystal resonator of the twin sensor type has, and nor do they mention a solution to these problems.